1. Field of the Invention
The present invention relates to a high strength welded joint excellent in the cold cracking resistance of a weld metal and a high strength welded steel pipe excellent in the cold cracking resistance of a weld metal and suitable particularly for a line pipe.
2. Description of the Related Art
In recent years, as production technologies progress, it has become possible to produce high tensile strength steels of HT80 and HT100 classes by lowering the carbon equivalents thereof with the aim of improving weldability, particularly cold cracking susceptibility. In this light, a method for preventing cold cracking by lowering a carbon equivalent and applying high heat input welding capable of securing a higher efficiency than low heat input multi-layer welding which has so far been commonly adopted is disclosed in Japanese Unexamined Patent Publication No. H9-314379.
However, in the case of an ultrahigh strength steel pipe having a tensile strength of 900 MPa or more in particular, it has become difficult in accordance with higher strengthening of steel to prevent cold cracking of a weld metal susceptible to strength. To solve with the problem, a method for preventing cold cracking by lowering the strength of a weld metal and controlling the width of a weld zone in an appropriate range is disclosed in Japanese Unexamined Patent Publication No. 2001-1148. However, with such a method as to optimize the shape of a weld, not only restrictions on welding conditions increase but also the shapes of a groove and a product may possibly be limited.
Meanwhile, preheating treatment or postheating treatment is generally effective in preventing cold cracking of an ultrahigh strength weld metal. In this light, with regard to a high strength steel having a tensile strength of 750 MPa or more, a method that stipulates the conditions of preheating before welding and heat insulation after welding in order to secure a cooling time from after welding up to a temperature of 100° C. is disclosed in Japanese Unexamined Patent Publication No. 2003-33876. Further, methods for preventing cold cracking by changing the welding conditions of the primary and secondary layers are disclosed in Japanese Unexamined Patent Publication Nos. 2001-71176 and 2002-309336. However, the problems in the case of applying those methods to the production of actual structures and line pipes are that the application causes productivity to deteriorate and a production cost to increase.
An ultrahigh strength steel pipe whose strength and cold cracking resistance are improved by optimizing components and further increasing an oxygen amount without depending on such weld metal shapes and welding methods is disclosed in Japanese Unexamined Patent Publication No. H10-306348. Further, the development of a method for suppressing cold cracking caused by hydrogen trapping is promoted and, as an example thereof, a welding wire capable of preventing cold cracking by containing a specific amount of V in a weld metal and precipitating carbide is disclosed in Japanese Unexamined Patent Publication No. H7-303991. Furthermore, an ultrahigh strength steel pipe whose cold cracking resistance is improved by forming retained γ by 1% or more in an inner side weld metal is disclosed in Japanese Unexamined Patent Publication No. 2002-115032.
Nevertheless, the problem in a method of increasing an oxygen amount in a weld metal or a method of adding a large amount of V has been that the deterioration of low temperature toughness of the weld metal is hardly avoided. Another problem has been that a large amount of alloying elements such as Ni, etc. must be added in order to form retained γ of 1% or more and thus the production cost increases.